The present invention relates generally to gas turbine engines, and, more specifically, to cooling therein.
A typical gas turbine engine includes a multistage compressor that pressurizes air which is then mixed with fuel in a combustor for generating hot combustion gases. Energy is extracted from the gases in multiple turbine stages for powering the compressor and producing useful work by powering a fan in an aircraft turbofan application, or powering an output shaft for marine and industrial applications.
The hot combustion gases flow along various components of the engine, which in turn are typically cooled by using a portion of the pressurized air bled from the compressor. For example, the combustion gases are born in the combustor which is typically defined by radially outer and inner annular combustion liners. The liners are typically provided with film cooling holes extending therethrough through which the pressurized compressor bleed air is channeled for effecting thermally insulating films of cooling air downstream from the outlets thereof.
The combustor also includes air swirlers, baffles, and splashplates surrounding corresponding fuel injectors in the upstream dome of the combustor, and additional patterns of film cooling holes are provided in this region for component cooling.
The hot combustion gases are initially discharged into a high pressure turbine nozzle which includes a row of stator nozzle vanes. The vanes are hollow and include film cooling holes in the sidewalls thereof for discharging film cooling air over the outer surface of the vane airfoils.
The first stage turbine also includes a row of turbine rotor blades extending radially outwardly from a supporting rotor disk. Each blade includes a hollow turbine airfoil with various rows of film cooling holes extending through the sidewalls thereof for film cooling the outer surfaces thereof.
Surrounding the turbine blades is a turbine shroud suspended from a surrounding casing. The turbine shroud typically also includes film cooling holes extending therethrough for film cooling the radially inner surface thereof which surrounds the blade tips.
Film cooling holes are also found in other components of the typical gas turbine engine and are arranged in various patterns for promoting a film cooling blanket of air over the outboard surfaces thereof which bound the hot combustion gases. The film cooling holes are typically arranged in linear rows, with the rows being spaced laterally apart for distributing the film cooling air as required for accommodating the local heat loads from the combustion gases.
The configuration, quantity, and pattern of the film cooling holes are specifically tailored for the expected heat load which varies from component to component and over the outboard surface of the individual component. A major objective is to minimize the amount of film cooling air bled from the compressor which is therefore not used in the combustion process and reduces efficiency of the engine.
However, performance of the film cooling holes is affected by the specific geometry thereof and the local conditions in the specific components including the differential pressure or pressure ratio between the outboard and inboard sides of the film cooling holes, and the velocity and pressure distribution of the combustion gases over the outboard surfaces.
The typical film cooling hole is tubular or cylindrical and manufactured by laser drilling for example. The film cooling hole is inclined through the component wall, with an inlet on the inboard side of the wall and an outlet on the outboard side of the wall. Each inclined film cooling hole therefore discharges a local jet of cooling air under a corresponding differential driving pressure across the component wall and with a corresponding discharge velocity through the hole outlet.
The blowing ratio of the differential pressure acting across the component wall affects the tendency of the discharged film cooling air jet to separate or blow-off from the outboard surface which is undesirable. The typical film cooling hole has a shallow inclination angle of about 30 degrees to ensure that the discharged film cooling air remains attached to the outboard surface and forms a film which extends downstream therealong in the aft direction corresponding with the predominant direction of the combustion gas flow.
Each row of film cooling holes has a specific pitch spacing between the centerlines of adjacent holes so that the separate jets of cooling air spread laterally in the downstream direction from the common row for promoting a continuous film of cooling air both laterally and aft along the component wall.
As the film cooling air flows aft from the film cooling hole outlet its cooling effectiveness diminishes as it begins to mix with the combustion gases flowing thereover. Accordingly, additional rows of film cooling holes are typically used and spaced transversely apart in the downstream direction for reenergizing the film cooling air from the preceding row of film cooling holes and ensuring effective film cooling air coverage over the outboard surface of the component for thermally insulating the component from the hot combustion gases.
Another form of film cooling hole is the diffusion hole which has various configurations in the art. In the diffusion hole the outlet portion thereof diverges or increases in flow area in the downstream aft direction from the upstream inlet for reducing the discharge velocity therefrom. An exemplary diffusion hole has a trapezoidal outlet with side edges which diverge at a suitably small diffusion angle, and an inner land which blends with the component outboard surface at a shallower inclination angle than the nominal inclination angle through the inlet portion of the hole.
In this way, the typical diffusion hole is effective for laterally spreading the discharged cooling air jet and locally enhancing film cooling performance.
In view of the complexity of the diverging diffusion film cooling holes, they are typically manufactured by electrical discharge machining (EDM) which requires a specially configured EDM electrode matching the desired configuration of the diffusion hole. EDM hole drilling is substantially more expensive than typical laser drilling of the cylindrical film cooling holes, and in view of the large number of film cooling holes typically found in an individual component, substantially increases the cost of manufacturing.
A typical turbine component such as a turbine rotor blade or nozzle vane may use various rows of cylindrical film cooling holes and shaped diffusion holes in different regions thereof for maximizing film cooling performance in the local regions of the blade subject to different thermal loads during operation. Rows of cylindrical film cooling holes and rows of diffusion holes have corresponding pitch spacing and have different performance and effectiveness both laterally from the row of holes and in the downstream aft direction. In either configuration, the effective coverage and density of the holes is correspondingly different, and correspondingly affects film cooling air performance, as well as the cost of manufacture due to the different complexity thereof.
Accordingly, it is desired to provide an improved film cooling arrangement for gas turbine engine components.